Fiber Bragg gratings are used for pressure sensing where the strain in the material is translated to a wavelength shift of a Bragg reflection. Polarization maintaining fibers are also used in pressure sensors including uni-axial pressure sensors where the wavelength shift of a Bragg reflection at one polarization differs from the wavelength shift in a second, orthogonal polarization. A single fiber can incorporate multiple sensors realized with Bragg gratings that reflect at different wavelengths and optionally multiple Bragg gratings reflecting at the same wavelength are incorporated in the fiber and differentiated by the “time of flight” of the reflections induced by these gratings when illuminated by an optical light pulse at the input to the fiber.
Uni-axial pressure sensors rely on the bi-refringence of glass optical fibers under uni-axial pressure. This means that the wavelength shift of Bragg reflectors incorporated in such fibers due to pressure applied differs for polarizations aligned with the pressure axis or orthogonal to the pressure axis. Regular single mode fiber is not polarization preserving, this means that light launched into the fiber in a particular polarization state does not remain in that state as it easily couples to modes in other polarization states that have nearly the same propagation constant. Index fluctuations and mechanical deformation of the fibers provide an effective coupling mechanism such that the input polarization state is typically lost within a small distance such as a few meters or less. Polarization maintaining fibers have a built-in stress in the fiber such that there are two orthogonal polarization states (typically linear) that have a significant difference in propagation constant such that light does not couple easily between modes in these two polarization states. Therefore light launched in one of the states remains in that state and fiber Bragg gratings can then be interrogated in a particular polarization state. Aligning the axis of the fiber with a pressure axis to be sensed allows the measurement of uni-axial pressure. However even when no pressure is applied the two propagation constants of the fiber differ and therefore two different Bragg reflections wavelengths (one of each orthogonal polarization) will occur where the exact wavelength difference depends on the amount of bi-refringence built into the fiber. It would be preferable to have a sensor that does not have a built in shift of the Bragg reflections and only shows a shift when uni-axial pressure is applied.